1. Field of the Invention
The present invention relates to information recording media on which information is recorded, erased, rewritten, and reproduced optically and electrically, methods for producing the same, and methods for recording/reproducing information thereon.
2. Description of the Prior Art
There are phase-changeable information recording media as information recording media on which information is recorded, erased, rewritten and reproduced using a laser beam. Information is recorded, erased, and rewritten on the phase-changeable information recording media utilizing the phenomenon that a recording layer is changed reversibly between a crystalline phase and an amorphous phase. In general, when recording information, the recording layer is melted and cooled abruptly with irradiation of a laser beam so that the irradiated portion is changed to an amorphous phase. On the other hand, when erasing information, the recording layer is irradiated with a laser beam having a lower power than that for recording to warm the recording layer and cool it gradually, so that the irradiated portion is changed to a crystalline phase. Therefore, in the phase-changeable information media, it is possible to record or rewrite new information while erasing recorded information by irradiating the recording layer with a laser beam that changes its power between a high power level and a low power level (see xe2x80x9cBasis and Application of Optical Disk Storagexe2x80x9d by Tsunoda Yoshito et al., The Institute of Electronics, Information and Communication Engineers, 1995, Chapter 2).
In recent years, various techniques have been under research to provide a large capacity to information recording media. For example, a technique for achieving high density recording by using a blue-violet laser having a short wavelength or using a thin substrate on the side from which a laser beam is incident and a lens having a large numerical aperture NA to reduce the spot diameter of the laser beam is under research. Another technique under research is such that using information recording media provided with two information layers, the two information layers are recorded/reproduced with a laser beam incident from one of the two information layers (see JP 12-36130 A). This technique, which uses two information layers, can achieve about twice the recording density.
In the information recording medium including two information layers on/from which information is recorded/reproduced from one side (hereinafter, may be referred to as a xe2x80x9ctwo-information-recording-layer medium), a laser beam transmitted through the information layer on the laser beam incident side (hereinafter, referred to as a xe2x80x9cfirst information layerxe2x80x9d) is used to record/reproduce information on/from the information layer on the side opposite to the laser beam incident side (hereinafter, referred to as a xe2x80x9csecond information layerxe2x80x9d). Therefore, it is preferable that the transmittance of the first information layer is at least 40%. On the other hand, it is desired that the second information layer has a high recording sensitivity (recording marks can be recorded even with a low power laser beam) with respect to the recording characteristics, and has a high reflectance with respect to the reproduction characteristics.
In order to achieve 40% of the transmittance of the laser beam through the first information layer, the first recording layer is required to have a small thickness of about 6 nm. However, if the recording layer is thin, the number of crystalline nuclei formed is small when crystallizing the recording layer. In addition, the distance over which atoms can move is short. Consequently, the crystallization rate of a thinner recording layer among recording layers made of the same material tends to be relatively lower. Therefore, as the recording layer is thinner, it is more difficult to form the crystalline phase, so that the erasure ratio is reduced.
Conventionally, as a material (phase-changeable material) of the recording layer, GeSbTe based materials having a high crystallization rate, excellent repeated rewriting performance and high reliability have been used. Using these materials, optical disks for computer data recording or optical disks for video recording have been commercialized. Among the GeSbTe based materials, quasi-binary compositions on the GeTe-Sb2Te3 line have the highest crystallization rate. The inventors of the present invention conducted recording/reproducing tests, using a red laser having a wavelength of 660 nm. The results are that in recording with a high linear velocity of 9 m/s, a satisfactory erasure ratio of 30 dB was obtained even if the recording layer made of GeTe-Sb2Te3 was as thin as 6 nm. This technique led to the feasibility of the two-information-recording-layer medium using a red laser.
Furthermore, information recording media in which a phase change is caused in the recording layer made of a phase changeable material by applying current have been under research. In these information recording media, the recording layer is interposed between two electrodes. In these information recording media, when current is allowed to flow gradually through the recording layer in an amorphous phase, the recording layer is changed to a crystalline phase at a certain threshold current, and the electrical resistance is dropped abruptly. Furthermore, the recording layer can return to the amorphous phase having a high resistance by applying a large current pulse having a short pulse width to the recording layer in the crystalline phase to melt and cool the recording layer abruptly. The difference in the electrical resistance can be detected easily by regular electrical means, and therefore such a recording layer allows a rewritable information recording medium to be obtained.
In order to provide a large capacity to information recording media, two-information-recording-layer media on which information is recorded/reproduced with a blue-violet laser having a short wavelength is desired for practical use. The spot diameter of a laser beam can be reduced by using a short wavelength laser beam or using a lens having a large numerical aperture and thus higher density recording can be achieved. For recording with a short wavelength laser beam, an information recording medium that allows a small recording mark to be formed in a satisfactory shape is required. When a blue-violet laser is used, the time during which the recording layer is irradiated with the laser beam is relatively short. Therefore, in order to form small recording marks, the recording layer is required to be formed of a material having a high crystallization rate. Furthermore, the recording layer is required to be formed of a material having a large change in the optical characteristics between the crystalline phase and the amorphous phase in order to obtain a sufficient signal amplitude even from small recording marks
In the experiments of the inventors of the present invention, when a conventional two-information-recording-layer medium using a red laser was used as an information recording medium for blue-violet laser, recording marks formed in the first information layer and the second information layer were small, and thus a sufficient signal amplitude was not obtained. Regarding the first information layer, when the thickness of the recording layer was made about 6 nm to ensure a sufficient transmittance, the erasure ratio was an insufficient value of less than 15 dB. The experiments of the inventors of the present invention confirmed that a large signal amplitude can be obtained by increasing the ratio of GeTe in a quasi-binary composition on the GeTe-Sb2Te3 line. However, as the ratio of GeTe is higher, the melting point of the material tends to be higher. Therefore, as the ratio of GeTe is higher, the laser power (recording power) required for forming an amorphous phase is larger. The output of the currently available blue-violet laser is smaller than that of a red laser. Therefore, when a composition having a large ratio of GeTe was used for the second information layer on which information is recorded/reproduced with a laser beam transmitted through the first information layer, the recording power was insufficient and thus the saturated signal amplitude was not obtained.
Thus, in the two-information-recording-layer medium for use with a blue-violet laser, it is important to ensure a high erasure ratio of the first information recording layer and a high recording sensitivity of the second information recording layer.
For practical use of the two-information-recording-layer medium for use with a blue-violet laser, the first information is required to have a high transmittance and good recording and erasing performance, and the second information recording layer is required to have a high reflectance, a high recording sensitivity and good recording and easing performance. Therefore, for the practical use of the medium, it is necessary to consider materials for the first and second recording layers and the constitution of the first and second recording layers.
For materials for the recording layers in which a phase change is caused by applying current, materials containing Te as the main component is in practical use. However, conventional materials require a long time in the order of microseconds for crystallization. An electrically phase-changeable information recording medium provided with two recording layers and having good recording and erasing performance has not been in practical use yet.
Therefore, with the foregoing in mind, it is an object of the present invention to provide an information recording medium with two recording layers and having good recording and erasing performance, a method for producing the same and a method for recording/reproducing information thereon.
An information recording medium of the present invention includes a first information layer and a second information layer. The first information layer includes a first recording layer in which a reversible phase change is caused between a crystalline phase and an amorphous phase by irradiation of a laser beam or the Joule heat generated by application of current. The second information layer includes a second recording layer in which a reversible phase change is caused between a crystalline phase and an amorphous phase by the irradiation of the laser beam or the Joule heat generated by the application of the current. The first recording layer is made of a first material. The second recording layer is made of a second material. The first material is different from the second material. According to the information recording medium of the present invention, good recording/reproducing characteristics can be obtained in each of the information layers.
In the information recording medium of the present invention, the first material may contain Ge, Sb, and Te, and the second material may contain Sb, Te, and at least one element M1 selected from the group consisting of Ag, In, Ge, Sn, Se, Bi, Au and Mn. According to this embodiment, with respect to a recording medium on which information is recorded with a laser beam (which may referred to as an xe2x80x9coptical information recording mediumxe2x80x9d in the following), an information recording medium including a first information layer having a high transmittance and a high erasure ratio and a second information layer having a high reflectance and high recording sensitivity can be obtained. In particular, this optical information recording medium is suitable for high density recording with a blue-violet laser. Furthermore, with respect to a recording medium on which information is recorded with current (which may referred to as an xe2x80x9celectrical information recording mediumxe2x80x9d in the following), a phase change between a crystalline phase and an amorphous phase easily can be caused selectively in the first recording layer, the second recording layer or both of them.
In the information recording medium of the present invention, the first material may be a material represented by a composition formula: GeaSbbTe3+a, where 0 less than axe2x89xa610 and 1.5xe2x89xa6bxe2x89xa64. This embodiment can provide good recording/reproducing characteristics, even if the first recording layer is very thin.
In the information recording medium of the present invention, the first material may be a material represented by a composition formula: (Ge-M2)aSbbTe3+a, where M2 is at least one element selected from the group consisting of Sn and Pb, and 0 less than axe2x89xa610 and 1.5xe2x89xa6bxe2x89xa64. According to this embodiment, Sn or Pb substituted for Ge of a Ge-Sb-Te ternary composition improves the crystallization performance, and thus a sufficient erasure ratio can be obtained, even if the first recording layer is very thin.
In the information recording medium of the present invention, the first material may be a material represented by a composition formula: (GeaSbbTe3+a)100xe2x88x92cM3c, where M3 is at least one element selected from the group consisting of Si, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Se, Zr, Nb, Mo, Ru, Rh, Pd, Ag, In, Sn, Ta, W, Os, Ir, Pt, Au and Bi, and 0 less than axe2x89xa610, 1.5xe2x89xa6b xe2x89xa64, and 0 less than cxe2x89xa620. According to this embodiment, the element M3 added to the Ge-Sb-Te ternary composition increases the melting point and the crystallization temperature of the recording layer, and thus the thermal stability of the recording layer can be improved.
In the information recording medium of the present invention, the second material may be a material represented by a composition formula: (SbxTe100xe2x88x92x)100xe2x88x92yM1y, where 50xe2x89xa6xxe2x89xa695 and 0 less than bxe2x89xa620. According to this embodiment, since the melting point of the second recording layer is low and the difference in the refractive index between the crystalline phase and the amorphous phase is large, a second information layer having high recording sensitivity and a large difference in the reflectance between the crystalline phase and the amorphous phase can be obtained.
In the information recording medium of the present invention, in the first and second recording layers, a reversible phase change may be caused by the irradiation of a laser beam. The first information layer may be disposed closer to the side from which the laser beam is incident than the second information layer. The melting point of the second material may be lower than that of the first material. This embodiment can provide a second information layer having high sensitivity.
In the information recording medium of the present invention, in the first and second recording layers, a reversible phase change may be caused by the irradiation of a laser beam. The first information layer may be disposed closer to the side from which the laser beam is incident than the second information layer. This embodiment can provide an optical information recording medium.
In the optical information recording medium, the thickness of the first recording layer may be 9 nm or less. This embodiment increases the transmittance of the first information layer, which makes it easy for laser light to reach the second recording layer in an amount necessary for recording/reproducing the second information layer.
In the optical information recording medium, the thickness of the second recording layer may be in the range of 6 nm to 15 nm. This embodiment can provide the second recording layer with particularly high recording sensitivity. When the thickness is 6 nm or more, the light absorption amount in the recording layer can be large. When the thickness is 15 nm or less, the volume of a portion melted to form a recording mark can be small, so that the recording sensitivity can be prevented from degrading.
In the optical information recording medium, the transmittance Tc (%) of the first information layer when the first recording layer is in a crystalline phase and the transmittance Ta (%) of the first information layer when the first recording layer is in an amorphous phase satisfy 40xe2x89xa6(Tc+Ta)/2 with respect to the laser beam having a wavelength in the range of 390 nm or more to 430 nm or less. This embodiment can provide good recording and erasing characteristics to the second information layer.
The optical information recording medium of the present invention may further include an optically separating layer disposed between the first information layer and the second information layer. The first information layer further includes a first substrate, a first lower protective layer, a first upper protective layer, and a first reflective layer. The second information layer may further include a second lower protective layer, a second upper protective layer, a second reflective layer, and a second substrate. The first substrate, the first lower protective layer, the first recording layer, the first upper protective layer, the first reflective layer, the optically separating layer, the second lower protective layer, the second recording layer, the second upper protective layer, the second reflective layer, and the second substrate may be disposed in this order from the side from which the laser beam is incident. According to this embodiment, with respect to the first and second information layers, the reflectance, the recording sensitivity, the erasing sensitivity, and the transmittance (especially of the first information layer) of each of the information layer can be optimized in accordance with the recording, erasure and reproduction conditions. Furthermore, the optical separating layer can separate the first information layer and the second information layer optically.
The optical information recording medium may further include a transparent layer disposed between the first substrate and the first lower protective layer.
The optical information recording medium may further include an interface layer disposed at at least one interface selected from the group consisting of an interface between the first lower protective layer and the first recording layer and an interface between the first upper protective layer and the first recording layer. The optical information recording medium may further include an interface layer disposed at at least one interface selected from the group consisting of an interface between the second lower protective layer and the second recording layer and an interface between the second upper protective layer and the second recording layer. The optical information recording medium may further include an interface layer disposed at at least one interface selected from the group consisting of an interface between the first upper protective layer and the first reflective layer and an interface between the second upper protective layer and the second reflective layer. According to these embodiments including the interface layer, atomic diffusion between the adjacent layers can be prevented, and an information recording medium having particularly high characteristics and reliability can be obtained.
The optical information recording medium may further include a transmittance adjusting layer for adjusting the transmittance of the first information layer between the first reflective layer and the optically separating layer. This embodiment can provide the first information layer with a particularly high transmittance.
The optical information recording medium may further include an interface layer disposed between the first reflective layer and the transmittance adjusting layer. According to this embodiment, atomic diffusion between the first reflective layer and the transmittance adjusting layer can be prevented, and an information recording medium having particularly high reliability can be obtained.
In the optical information recording medium, the thickness of the first substrate may be in the range of 10 xcexcm to 800 xcexcm. According to this embodiment, by changing the numerical aperture (NA) of the objective lens, the length, the width and the interval of recording marks can be optimized in accordance with the shape of the groove of the first substrate and recording, erasure and reproduction conditions. For example, when the thickness of the first substrate is 100 xcexcm, good recording and erasing performance can be obtained with NAxe2x89xa60.85. When the thickness of the first substrate is 600 xcexcm, good recording and erasing performance can be obtained with NAxe2x89xa60.6.
In the optical information recording medium, the thickness of the second substrate may be in the range of 400 xcexcm to 1300 xcexcm. According to this embodiment, by changing the NA of the objected lens, the length, the width and the interval of recording marks can be optimized in accordance with the shape of the groove of the second substrate and recording, erasure and reproduction conditions. It is preferable to select the thicknesses of the first and second substrate so that the thickness of the information recording medium is about 1200 xcexcm. For example, when the thickness of the first substrate is 100 xcexcm, it is preferable that the thickness of the second substrate is 1100 xcexcm. When the thickness of the first substrate is 600 xcexcm, it is preferable that the thickness of the second substrate is 600 xcexcm.
The information recording medium of the present invention may further include first and second electrodes. In the first and second recording layers, a reversible phase change may be caused by the application of the current. The first recording layer, the second recording layer and the second electrode may be laminated over the first electrode in this order. This embodiment can provide an electrical information recording medium.
The electrical information recording medium may further include an intermediate electrode disposed between the first recording layer and the second recording layer. According to this embodiment, atomic diffusion between the first recording layer and the second recording layer can be prevented, so that the repetition characteristics and the reliability can be improved. Furthermore, according to this embodiment, current can be applied to either one of the first recording layer and the second recording layer.
According to another aspect of the present invention, a method for producing an information recording medium including a first information layer and a second information layer is provided. The method includes the processes of: (a) forming the first information layer; and (b) forming the second information layer. The first information layer includes a first recording layer in which a reversible phase change is caused between a crystalline phase and an amorphous phase by irradiation of a laser beam or the Joule heat generated by application of current. The second information layer includes a second recording layer in which a reversible phase change is caused between a crystalline phase and an amorphous phase by the irradiation of the laser beam or the Joule heat generated by the application of the current. The process (a) includes forming the first recording layer with a base material containing Ge, Sb, and Te. The process (b) includes forming the second recording layer with a base material containing Sb, Te, and at least one element M1 selected from the group consisting of Ag, In, Ge, Sn, Se, Bi, Au and Mn. According to this embodiment, the information recording medium of the present invention can be produced easily.
In the production method of the present invention, the first and second recording layer may be formed by sputtering using sputtering gas containing argon gas or krypton gas. This sputtering gas may further contain at least one gas selected from the group consisting of nitrogen gas and oxygen gas. This production method can provide information layers having excellent repeated recording performance.
In the production method of the present invention, the thickness of the first recording layer may be 9 nm or less. In the process (a), the first recording layer may be formed at a film formation rate in a range of 0.1 nm/sec. to 3 nm/sec. According to this embodiment, an optical information recording medium including a first recording layer with little variation in the thickness can be produced with high productivity.
In the production method of the present invention, the thickness of the second recording layer may be in the range of 6 nm to 15 nm. In the process (b), the second recording layer may be formed at a film formation rate in a range of 0.3 nm/sec. to 10 nm/sec. According to this embodiment, an optical information recording medium including a second information layer with high recording sensitivity can be produced with high productivity.
In the production method of the present invention, the process (b) may be performed before the process (a). After the process (b) and before the process (a), the method may further include the process (c) of forming an optically separating layer over the second information layer. In the process (a), the first information layer may be formed over the optically separating layer.
A first method of the present invention for recording/reproducing an information recording medium is a method for recording/reproducing the information recording medium of the present invention. With respect to the first information layer of the information recording medium, information is recorded/reproduced with a laser beam incident from the side of the first information layer. With respect to the second information layer of the information recording medium, information is recorded/reproduced with the laser beam that has passed through the first information layer. The wavelength of the laser beam is in the range of 390 nm or more and 430 nm or less. According to the first method for recording and reproduction, information can be recorded/reproduced on/from the first and second information layers of the optical information recording medium in a high density and with good reliability.
In the first method for recording and reproduction, the linear velocity of the information recording medium when recording/reproducing information may be in the range of 3 m/sec. or more and 30 m/sec. or less.
In the first method for recording and reproduction of the present invention, the laser beam may be focused by an objective lens, and the numerical aperture NA of the objective lens may be in the range of 0.5 or more and 1.1 or less. According to this embodiment, the length, the width and the interval of recording marks can be optimized in accordance with the thickness or the shape of the groove of the first and second substrates and recording and reproduction conditions.
A second method of the present invention for recording/reproducing an information recording medium is a method for recording/reproducing the information recording medium of the present invention. In the first and second recording layers of the information recording medium, a reversible phase change is caused between a crystalline phase and an amorphous phase by the Joule heat generated by application of current. An amplitude Ic, a pulse width tc, an amplitude Ia1, a pulse width ta1, an amplitude Ia2 and a pulse width ta2 satisfy the relationships: Ic less than Ia2 less than Ia1 and ta1xe2x89xa6tc or ta2 less than tc, wherein a current pulse with the amplitude Ic and the pulse width tc is applied to the first or second recording layer to change the first or second recording layer from an amorphous phase to a crystalline phase; a current pulse with the amplitude Ia1 and the pulse width ta1 is applied to the first recording layer to change the first recording layer from a crystalline phase to an amorphous phase; and a current pulse with the amplitude Ia2 and the pulse width ta2 is applied to the second recording layer to change the second recording layer from a crystalline phase to an amorphous phase. According to this second method for recording and reproduction, in an electrical information recording medium, a phase change between a crystalline phase and an amorphous phase easily can be caused selectively in the first recording layer, the second recording layer or both of them. In the electrical information recording medium of the present invention, the phase change causes a change in the electrical resistance, and therefore this can be used as an element of a changeable and programmable circuit.
As described above, according to the information recording media of the present invention and the method for producing the same, information recording media including two recording layers and having good recording and erasing performance can be obtained.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.